(1) Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer, copier, facsimile machine and the like, in particular relating to an image forming apparatus in which a small volume sheet tray unit accommodated therein can be replaced as required by a duplex printing tray unit or a large volume sheet tray unit.
(2) Description of the Prior Art
In conventional image forming apparatuses such as copiers, the sheet tray for holding sheets or the original table portion for placing an original cannot be made smaller than a maximum size of sheets or original to be used, while various kinds of functional parts inside the image forming apparatus have been developed to be compact. As a result, the size of the footprint of the image forming portion including an image forming means and a fixing means when viewed from above has become approximately equal to the size of the sheet tray of the maximum-sized sheets.
There are demands for provision of multiple sheet trays which allow simultaneous setting of various kinds of sheets and for reducing the floor space (to be referred to hereinbelow as footprint) which an image forming apparatus occupies when it is installed.
For these reasons, image forming apparatuses have been developed to have a layered structure: that is, the scanner portion for scanning the original, the image forming portion (including an image forming means and fixing means) for forming an original image onto a sheet of paper, and a sheet tray portion for accommodating sheets are arranged in this sequential order from the top to the bottom.
In such image forming apparatuses having a layered structure, functional parts for various functions required for image forming, such as a scanner portion, image forming portion, fixing portion, sheet storage portion, sheet conveying portion and the like are formed as individual functional units so that the functions of the image forming apparatus can be easily changed by selecting the combination of the functional units to be assembled into a main frame for accommodating functional units.
For example, as shown in FIG. 1, the image forming apparatus in this case is made up of a machine body 11 having a scanner portion and an image forming portion and an accommodation for accommodating a sheet storage unit 12.
In an image forming apparatus of this type, it is possible to use a common frame for the sheet storage unit when a size of sheets is changed into another and hence inhibit the increase in cost of the product due to increase in the number of parts and the increase in the cost for dies. Nevertheless, if a duplex printing function needs to be added to an already purchased image forming apparatus, the duplex printing tray, which will have a different height, is forced to be placed under the sheet storage unit, and therefore the conveyance path becomes longer, giving rise a problem in that the job speed for duplex printing is degraded.
As a means for solving this problem, it is disclosed in Japanese patent Application Laid-Open Hei 5 No. 270,714 that a device for inverting and conveying sheets for duplex printing is detachably provided between the image forming unit inside the image forming apparatus body and the first sheet storage tray. This apparatus is configured so that this sheet inverting and conveying device can be detached from the image forming apparatus body and a second sheet storage tray having the same size as the sheet inverting and conveying device can be attached in place.
However, in the image forming apparatus of this type, the size of the sheet storage unit to be attached was limited, and therefore it was impossible by any means to attach a sheet storage unit greater than the original one.
An image forming apparatus shown in FIG. 2 is composed of a machine body 13 integrally having a scanner portion, an image forming portion and a plurality of sheet storage units 14a, 14b and 14c accommodated in respective frames 15a, 15b and 15c and stacked under machine body 13, separably from each other. Therefore, it is possible to easily change the variety and number of sheet storage units which are stacked under the upper unit (Japanese Utility Model Publication Hei 8 No. 1,226).
This image forming apparatus disclosed in Japanese Utility Model Publication Hei 8 No. 1,226, however, has a problem. That is, if various types of sheet storage units having different heights i.e., small volume sheet storage trays, large volume sheet storage trays, duplex printing trays, etc., are set, different sizes of large accommodation frames for the various storage units are needed and the dies for molding those accommodation frames increase the product cost.